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Thermoplastic polymers are often reinforced by adding short fibers to improve mechanical properties including Young's modulus and tensile strength of the polymers. In many engineering applications, energy absorbing characteristics in such particulate polymers is known to be a very important property to be considered in composite designs, and meanwhile debonding at the interface between fiber and matrix in the composites may affect the energy absorption properties. Here, the focus of this study is to employ a semi-empirical approach to determine the debonding stress and investigate the effect of the debonding stress on energy absorbing properties of short glass fiber reinforced polycarbonate composites. Glass short fiber reinforced polycarbonate composites are fabricated via a solution mixing technique. Tensile testing and acoustic emission measurement are simultaneously performed for the polycarbonate composites. The test results including toughness are compared for the composites over neat polycarbonate. Also the local debonding stress in the vicinity of each glass fiber in composites is estimated by combining modeling and experiments. A finite element model is developed to determine local debonding stress at the interface between the fiber and matrix. The local debonding stress appears to considerably affect the toughness of the composites.

Toughness of a polymer is a key material property for energy absorbing capability for various engineering applications. Significant effort has been made to improve toughness of a polymer and hence increase the energy absorbing capability; typically rigid-particles in thermoplastics or rubbery modifiers in a brittle polymer matrix. The focus of this study is to investigate toughening mechanisms of a thermoplastic polymer composite. Micron-size thermoplastic particle reinforced polycarbonate (PC) composite materials was fabricated via a solution mixing method. The mechanical properties of the polymer composites were characterized in tensile testing while the acoustic emission was monitored to assess the material failure modes during the tensile test. Substantial improvement in tensile toughness was observed for the polymer composites and the toughening mechanisms responsible for the improvement were identified and quantified for each contribution to the observation.

This study involves the investigation of spherically shaped filler diameter and interphase effects on the Young's modulus of micro and nano size silicon dioxide (SiO2) particle reinforced epoxy composite materials. Specifically, 10m and 80nm size SiO2particles and Epon 862 epoxy are chosen as fillers and a matrix material, respectively. While 10m and 80nm SiO2particles are dispersed in the epoxy through a direct shear mixing method, nano-composites are fabricated with hardener at desirable ratios. Both micro- and nano-composites are prepared at 2 different particle loading fractions for tensile testing. It is observed that the nano-composites show significant increase in Young's modulus over micro-composites, showing a linear increase as particle volume fraction increases. This could indicate that for nano-composites, the interphase region between the particle and matrix can considerably affect their mechanical properties. Here, we develop a finite element analysis (FEA) model to investigate the interphase effect on the Young's modulus of both micro- and nano-composites. This model demonstrates how to estimate the effective volume fraction of a particle as filler using a combined experimental/numerical approach. The effective volume fraction is shown to be important in predicting the mechanical response of nano-scale particles reinforced composite materials.

Recent studies show that continuously reinforced multi-walled carbon nanotubes (MWCNT) composite can have extraordinary mechanical properties. It was observed that the continuous MWCNT polymer composites exhibit both significant reinforcement and large damping capability in compressive loadings, which typically remain compromised. The damping property might result from buckling behavior of the MWCNT in composites. Here, this paper is to study the buckling response of carbon nanotubes (CNT) within a polymer matrix by using analytical models including Euler, Timoshenko and shell buckling models. Also, the modeling results are analyzed and compared to better understand the bucking behavior of the CNT in the composite and also investigate the effect of their aspect ratio (L/D) on buckling behavior. This study provides us with insight to better understand the structure-property relation for such continuous CNT polymer composites.

Recent advances in the production and availability of nanoscale materials has led to a significant interest in the use of nanoscale fillers in order to augment and tailor material performance in nanostructured composites. A specific area of interest is the use of high aspect ratio fillers, such as carbon nanotubes (CNT) and carbon nanofibers (CNF) to augment the damping capacity of nanostructured composites. Previous work has shown the use of high aspect ratio fillers to significantly enhance the damping capacity at low frequency by more than 100%; however, the enhancement achieved has been predicated on strain levels in the composite. Our previous studies have indicated a strong strain dependent response in the nanostructured composites utilizing CNF to augment damping capacity. This is due, in part, to the random distribution of fiber orientations seen in the nanostructured composites. The random distribution of filler orientations is thereby relative to the load applied to the composite that results in a critical shear stress thresholds being surpassed at the nano scale, allowing the filler to slip relative to the matrix, resulting in frictional energy dissipation as heat and thereby inducing damping to the high aspect ratio filler nanostructured composite. In light of the promise this technology holds for use in engineered applications requiring specific damping performance, there remains a fundamental lack in understanding of the precise mechanisms and thereby a lack of ability to accurately predict material performance, which is limiting application of the technology. This study looks at the effect of the random filler orientation of CNF included composites and examines the viscoelastic response of the composite specifically investigating the effect of filler orientation relative to the loading direction and the effect of filler waviness. Furthermore, this study looks at the strain dependent nature of the viscoelastic response and develops an analytical modeling tool to look at the effect of the strain dependent viscoelastic response seen in previous studies with the aim of achieving a better fundamental understanding of the effect of filler orientation and the associated strain dependent nature of the viscoelastic response seen in high aspect ratio nano- filled composites.

The focus of this study was to experimentally investigate spherically shaped micron-size particles reinforced polymethyl methacrylate (PMMA) and polycarbonate (PC) polymer composites for improving energy absorbing capabilities such as toughness and low-velocity impact resistance. In this study, a solution mixing method was developed to fabricate both PMMA and PC polymer composites with spherically shaped micron-size polyamide- nylon 6 (PA6) particles inclusions. The morphology of the fracture surfaces of polymer composites was examined by using optical microscopy and scanning electron microscopy. Strain-rate dependent response of both PMMA and PC polymer composites was investigated by characterizing tensile and flexural properties. Low-velocity penetration testing was performed for both polymer composites and the key results observed for energy absorption capabilities are discussed in this study.